The present invention relates generally to an ink jet printing apparatus, and is specifically concerned with a hot melt ink jet printer in which an articulated arm is provided for connecting the ink reservoir to the print head so as to allow the print head to be positioned independently of the reservoir.
A conventional hot melt ink jet printing apparatus includes a heated reservoir assembly for melting a solid ink pellet and a print head for ejecting the melted ink. Normally, the print head is directly mounted to the reservoir assembly in a manner such that there is no relative movement between the reservoir assembly and print head. The print head has an array of nozzles, usually forming a straight line. When mounted on the reservoir assembly, this line of nozzles is at a slight angle with respect to the horizontal. As the object to be printed is conveyed in a horizontal direction past the print head, the nozzles are actuated in a sequential manner in order to form dot-matrix characters on the surface of the object. The angle formed by the line of nozzles relative to the horizontal defines the height of the printed characters.
Conventional hot melt ink jet printers with fixed print heads are not well suited to applications in which it is desired to print on a non-vertical surface of a horizontally moving object, or to move the object past the print head in a direction other than the horizontal. In some cases, it is possible to tip the entire ink jet apparatus so that the nozzles of the print head face the surface to be printed and line up properly with the direction of product movement. However, in instances where it is necessary to print on the top surface of the product, or where the product is being conveyed vertically during printing, this would require tipping the apparatus a full 90 degrees. Such tipping of the ink jet apparatus poses serious problems. Normally, the ink jet apparatus has one or more level sensors in the reservoir assembly for detecting when the ink is low and for shutting down the system to prevent de-priming. Tipping of the ink jet apparatus can set off these level sensors and cause a premature shutdown of the system. Furthermore, extreme tipping of the reservoir assembly can cause spillage of the ink if the print head is brought to for above or below the level of the reservoir. Such may also cause ink starvation or siphoning at the print head.
An additional disadvantage of conventional hot melt ink jet printing apparatus concerns the height of characters printed. In order to increase the printed character height, the angle formed by the array of ink jet nozzles relative to the horizontal must be increased in order to increase the effective vertical spacing between successive nozzles. However, because the print head is fixedly mounted to the reservoir assembly, the print head and reservoir must be tilted as a unit in order to achieve this result. In addition to the physical difficulty involved in repositioning the entire reservoir and print head assembly, there is the additional problem that the operation of the ink level sensors can be adversely affected whenever the orientation of the reservoir is changed. Therefore, with typical hot melt ink jet printing apparatus, only small changes in character height can ordinarily be obtained.